This invention relates to a fluidized bed reactor and method for operating same and, more particularly, to a fluidized bed reactor utilizing a strip-air system for reducing the heat content of, and removing the relatively fine particulate material from, the waste solids drained from the furnace section of the reactor while at the same time increasing the reactor's combustion efficiency.
Reactors, such as combustors, steam generators and the like, which utilize fluidized beds as their primary source of heat generation, are well known. In these arrangements, air is passed into the furnace section of the reactor and through a bed of particulate material contained therein which includes a mixture of a fossil fuel, such as coal, and an adsorbent, such as limestone, to adsorb the sulfur generated as a result of the combustion of the coal. The air fluidizes the bed and promotes the combustion of the fuel.
To improve the pollution characteristics of fluidized bed reactors, it is known to stage the combustion of the fuel by controlling the amount of oxygen in various regions of the fluidized bed. In general, the lower region of the fluidized bed is operated under fuel rich or substoichiometric conditions such that nitrogen oxides emissions are reduced. The upper region is then operated under oxygen rich or oxidizing conditions to complete the combustion of the fuel.
Each region of the fluidized bed is comprised of a homogenous mixture of particles of fuel and adsorbent, with a portion of the fuel particles being unburned, a portion being partially burned and a portion being completely burned; and a portion of the adsorbent being unreacted, a portion being partially reacted and a portion being completely reacted. The particulate material must be discharged from the system efficiently to accommodate the introduction of fresh fuel and adsorbent. To this end, a portion of the particulate material is usually passed from the lower region of the bed through a drain pipe to remove that portion from the reactor system.
It has been found, however, that the particle size distribution in a fluidized bed, an important operating parameter, can be effectively controlled by recirculating part of this removed particulate material back to the furnace section. This is often accomplished by blowing air through the removed particulate material to strip away and entrain the finer portions of the particulate material and returning them to the furnace section.
For example, in U.S. Pat. No. 4,829,912, a patent assigned to the same assignee as the present application and incorporated herein by reference, a method of controlling the particle size distribution in a fluidized bed reactor is disclosed in which the particulate material removed from the furnace section is passed through jets of air to entrain the finer portions of the removed particulate material by stripping them away from the larger solids and then recirculating these finer portions back to the furnace section. The non-stripped, nonrecirculated particulate material is passed to an ash handling system for removal from the reactor system. However, since this nonrecirculated particulate material has a temperature which exceeds the design temperature of common ash handling systems, the material must be cooled prior to its passage to the ash handling system. In these types of arrangements, the heat removed from the nonrecirculated particulate material can be put to productive use, such as to preheat combustion supporting gas or for reheat or superheat duty.
A stripper/cooler located adjacent the furnace section of the reactor can both recirculate the finer portions of the removed particulate material and cool the removed but nonrecirculated particulate material. In these types of arrangements, a first, or stripper, section of the stripper/cooler receives the particulate material from the lower region of the fluidized bed through a drain pipe. Air is blown through the stripper section to strip, or entrain, some of the finer portions of the particulate material which portions are then returned to the furnace section. The particulate material remaining in the stripper/cooler is then usually passed to a second, or cooler, section of the stripper/cooler where heat is removed from the particulate material by passing water or steam in a heat exchange relation to the particulate material or by blowing air through it before it is discharged to the ash handling system.
The stripper/cooler system just described is not without its drawbacks. For example, a significant portion of the particulate material removed from the furnace section of the reactor will be noncombusted fuel due to the usually substoichiometric conditions maintained in the lower region of the fluidized bed from which the particulate material is removed. This leads to less than optimal combustion efficiency for the reactor system since the removed noncombusted fuel is not recirculated to the fluidized bed due to its relatively large size. It is therefore discharged through the ash handling system.
Further, as the particulate material is removed from the furnace section, it takes heat with it reducing the available heat in the furnace and requiring a cooling system to enable the ash handling system to manage the material. Moreover, duct work is required to return the stripped particulate material to the furnace section.